Rapid recognition of invading pathogens by the host organism is crucial in mounting an effective immune response. Conserved structural features on pathogens termed PAMPs are recognized by the Toll-like cell surface receptors, which are part of the evolutionary conserved innate immune system. We had previously identified the activation of Interferon Regulatory Factor 3 (IRF3) and the subsequent the induction of Interferon Stimulated Genes (ISGs) as a novel signaling pathway that is initiated by TLR ligands. We have characterized this pathway as a major contributor to septic shock syndrome, but also found that the IRF-3 activation cascade is a target for bacterial toxins such a Bacillus anthracis Lethal Factor. Recently, we studied a novel family of LPS and interferon-induced nucleic acid binding proteins (schlafen = slfn) whose members appear to alter IRF3, but not NF:B mediated transcriptional responses towards TLR ligands. The proposed research is aimed towards investigating the function of the Slfn proteins in IRF3-mediated innate immune responses. We hypothesize that the longer members of the Slfn family act either as cytoplasmic sensors of foreign nucleic acids directly, or possibly as cofactors to such sensory proteins, whereas the medium and short cellular Slfn isoforms as well as the viral Slfn proteins function to attenuate the innate immune response. Experiments are proposed to characterize the specificity of Slfn function, to define their point-of-action in the IRF3 activation cascade, and to identify Slfn-interacting proteins. Results from these proposed studies will not only facilitate our understanding of the mechanism of IRF3 activation, but will also shed light on the role of Slfn proteins as novel modulators of the innate immune response under physiological and pathological processes. PUBLIC HEALTH RELEVANCE: IRF3 is one of the key mediators of TLR-induced responses. Lack of IRF3-activation leads to an immobilization of the immune response, yet attenuation of this pathway can be beneficial during septic processes. Thus, a detailed understanding of factors such as Slfn proteins that can modulate IRF3 activation is likely to provide novel targets for pharmacological intervention during infectious disease processes.